This invention relates to an automatic shoe clearance adjusting device in a shoe drum brake for use in vehicles such as automobiles and, particularly, to an automatic shoe clearance adjusting device of the type including a first strut member, having a hollow or cylindrical portion on one end, a second strut member having a male screw threaded portion thereon and engaging telescopically with the first strut member with one portion thereof being received in the cylindrical portion of the first strut member, an adjusting nut supported rotatably on the first strut member and engaging with the screw threaded portion of the second strut member, and means for rotating the adjusting nut on the second strut member when clearance (shoe clearance) between the brake shoes and the brake drum of the shoe drum brake exceeds a predetermined value.
In a conventional automatic shoe clearance adjusting device of the aforesaid type, the adjusting nut is rotated by a suitable member such as an adjusting lever rotatable by a hand brake lever when applying the hand brake, and the reverse rotation of the adjusting nut by the adjust lever is prevented by frictional force acting between the first strut member and the adjusting nut caused by the shoe returning force. Shoe return springs disposed between the shoes usually provide the shoe returning force, but the force will sometimes be reduced by frictional force in the hydraulic cylinder or frictional force between the shoes and the back plate whereby adjustment of the clearance is not effected properly.
To prevent such reverse rotation of the adjusting nut there has been proposed a pawl member engaging with the adjusting nut, but there is a shortcoming in this arrangement in that when the brake is applied under a severe condition the adjusting nut rotates too much in applying the brake due to thermal expansion of the brake drum, thus causing over-adjustment.